1. Field of the Invention
The present invention relates to a developing apparatus that develops a latent image formed on an image bearing member by a developer so as to convert the latent image into a visible image, a process cartridge provided with the developing apparatus, and an image forming apparatus, such as a copier and a printer, which is provided with the developing apparatus.
2. Description of Related Art
Conventionally, an electrophotographic image forming apparatus, which optically scans an original image portion that is on an outer circumferential surface of a uniformly electrified photosensitive drum (an image bearing member) so as to form an electrostatic latent image, and converts the electrostatic latent image into a visible image by using a toner that is a colored resin, is known. Such an image forming apparatus is capable of forming an image at a high speed, and thus has been used widely for digital printers, copiers and the like.
In recent years, there have been increasing demands particularly for forming color images. As electrophotographic image forming apparatuses, apparatuses for forming full color images composed of toner images of four colors: yellow (Y); magenta (M); cyan (C); and black (Bk) also have been realized. Recently, there have been increasing demands to reduce the size of the electrophotographic image forming apparatus bodies to save space, in addition to the demands for enhancing their printing speeds and image qualities.
Moreover, due to the establishment of fixing techniques by lowering melting points of toners and utilizing an induction heating method, electrophotographic image forming apparatuses that can instantly fix toners on recording sheets and can start up quickly have been developed widely.
On the other hand, a two-component developer containing a toner and a carrier has been widely used for developing electrostatic latent image to provide high image quality and low operating costs. In this case, an electrostatic latent image that is formed on a surface of the photosensitive drum is developed using a developing apparatus that is provided with a development sleeve having magnets disposed therein, by rubbing the developer against the surface of the photosensitive drum with a magnetic brush.
In such a developing apparatus, two developer transporting screws are arranged horizontally in the developing apparatus, and a developer is circulated between these two developer transporting screws, whereby the toner and the carrier can be mixed uniformly, and frictional charging of the toner can be performed sufficiently.
The sufficiently stirred developer is supplied to the development sleeve by a magnetic force of the magnets, and is transported according to rotation of the development sleeve. The transported developer passes through a developer regulating member, whereby an amount of the developer on the development sleeve is regulated to be a proper amount, and a uniform layer of the developer is formed on the development sleeve. The magnetic brush of the developer that is born by the development sleeve is in contact with the photosensitive drum that rotates at a development portion, and then the electrostatic latent image on the photosensitive drum is developed.
FIGS. 9A to 9C show a developer transporting screw for transporting a developer in the prior art, more specifically, FIG. 9A is an external perspective view of the developer transporting screw, FIG. 9B is a cross-sectional view of the developer transporting screw cut by a plane passing through a rotational center line, and FIG. 9C is a view describing a state of the developer transporting screw transporting a developer. In FIGS. 9A to 9C, reference numeral 33 denotes a rotational center line, reference numeral 34 denotes a rotation shaft, reference numeral 35 denotes a vane that winds around the rotation shaft 34 in a spiral form, and reference numeral 41 denotes a developer. Reference character θ′ denotes an angle between: a vane surface on a developer transport direction side of the vane 35; and the rotational center line 33, and reference character θ denotes an angle between: a vane surface on other side of the developer transport direction of the vane 35; and the rotational center line 33. When the vane surface on the developer transport direction side of the vane 35 is closer to perpendicular with respect to the rotational center line 33 (the angle θ′ is closer to 90°), the force required to transport the developer is larger, and thus the angles θ and θ′ usually are set to be within a range from about 70° to about 85° (θ=θ′), considering the draft angle of a forming die and the like.
As shown in FIG. 9, in accordance with the rotation of the developer transporting screw, a developer 41 receives a pushing force in a traveling direction by the vane 35, and thus the developer 41 leans toward the vane surface on the developer transport direction side of the vane 35. Further, since the amount of the developer 41 decreases as the distance from the vane 35 increases, the developer 41 between each of the vanes 35 is transported in a state shown in FIG. 9C. Thus, from a distribution of the developer 41 in a longitudinal direction of the developer transporting screw, it is found that a part with a large amount of the developer 41 and a part with a small amount thereof are formed in accordance with a pitch of the vane 35 (hereinafter, this deviation of the developer is called a “maldistribution”).
Since the developer transporting screw transports the developer 41 in the longitudinal direction by its rotation, this unevenness of the distribution of the developer 41 results in unevenness of density of an image in an oblique direction (hereinafter, this poor image is called “unevenness by the screw pitch”).
Such unevenness by the screw pitch easily is generated, in particular, when using a small-sized developing apparatus for a two-component developer in which a development sleeve and a developer transporting screw for transporting a developer are arranged close to each other. In order to prevent the maldistribution of the developer in the screw vane pitch, a developer transporting screw in which an angle between: a vane surface on a developer transport direction side of a vane; and a rotational center line is set to be 60° or less (for example, see JP 2004-117507 A), and a developer transporting screw provided with a bulk-increasing vane between transporting vanes, the bulk-increasing vane having a smaller angle between: a vane surface on a developer transport direction side thereof, and a rotational center line than that of the transporting vane (for example, see JP 2004-151326 A) are suggested.
In a developing apparatus using each of the above-described developer transporting screws, the developer transporting screw rotates so that the vane surface on the developer transport direction side of the vane generates a power to push the developer in a direction perpendicular to the rotation shaft, whereby this power is utilized so as to prevent the maldistribution of the developer.
However, the configurations according to the above-described JP 2004-117507 A and JP 2004-151326 A have problems described below.
It is known that, in the case of using the usual developer transporting screw, the height of the developer 41 decreases as the distance from the vane surface on the developer transport direction side of the vane 35 increases, as shown in FIG. 9C. Thus, as described in JP 2004-117507 A, in the case where the angle between: the vane surface on the developer transport direction side of the vane; and the rotational center line is set to be 60° or less, when the developer transporting screw is in a static state, the position of the vane surface becomes lower as the distance from a tip of the vane toward a downstream side of the developer transport direction increases, and thus the maldistribution of the developer remains in a position at a long distance from the tip of the vane. In particular, in an image forming apparatus that can start up quickly, which currently is becoming popular, a quick development is necessary. However, according to the configuration of the above-described JP 2004-117507 A, a long period of time is required from the start of the rotation of the developer transporting screw to eliminating the maldistribution of the developer. As a result, if developing shortly after the start of the rotation of the developer transporting screw, the maldistribution of the developer cannot be resolved, and the unevenness by a screw pitch remains on the image.
In addition, as described in JP 2004-151326 A, in the case of providing a bulk-increasing vane having a smaller angle between: the vane surface on the developer transport direction side thereof; and the rotational center line than that of the transporting vane, when the developer transporting screw is in a static state, the maldistribution of the developer is improved due to the presence of the bulk-increasing vane. However, since such a bulk-increase is achieved at a position where the height of the bulk of the developer is not the smallest, the maldistribution of the developer remains near the vane surfaces on the other side of the developer transport direction of the transporting vanes that are long distances from the bulk-increasing vane and are adjacent to each other. Therefore, in the case of JP 2004-151326 A, similarly to the case of JP 2004-117507 A, if developing shortly after the start of the rotation of the developer transporting screw, the maldistribution of the developer cannot be resolved, and the unevenness by the screw pitch remains on the image.
The present invention is provided to address the above-described problems. The inventors of the present invention have been resolving the maldistribution of a developer near an outer circumferential part of the screw in a static state and in a rotating state, by providing an bulk-increasing portion on the vane surface on the other side of the developer transport direction of the vane of the developer transporting screw, and utilizing a bulk-increasing effect obtained thereby. However, by providing the bulk-increasing portion between the vanes that are adjacent to each other, the amount of the developer to be transported by the developer transporting screw is decreased by the thus increased bulk, and thus an image density is decreased. The inventors of the present invention have conducted a keen study by examining: an angle between the vane of the developer transporting screw and the rotational center line of the screw; and an angle between the bulk-increasing portion and the rotational center line. A result of the study, they found conditions that enable resolution of the maldistribution of a developer near the outer circumferential part of the screw in a static state and in a rotating state, and enables securing an amount of a developer to be transported.